guzhao67“what's the meaning of "simple"?”
plain -- just one word -- unadorned by any grammatical machinery for creating the progressive or perfect aspects or the passive voice. It's a way of having a term that fits into the following paradigm.
simple perfect
progressive perfect progressive
passive passive perfect
passive progressive passive perfect progressive
Both the term
present and the term
past can precede any of the eight designators above.
The word
continuous can substitute for the word
progressive. They both mean the same thing.
Without the term
simple, one item in the paradigm would have no name!

CJ